The present invention relates to a method and an apparatus, for performing a counter-current mass exchange between two phases having different densities, notably between a liquid phase and a solid phase, such as an ion exchange resin, or between two non-miscible liquid phases. This invention is applicable to any liquid/solid or liquid/liquid exchange technique, and more particularly to the field of hydrometallurgy for recovering metal ions in a concentrated form from solutions for attacking ores, notably uranium ore, for instance.
Many methods have already been proposed in the art of ion exchange between a liquid phase to be purified and solid particles such as ion exchange resins, the liquid and the ion exchange resins flowing in counter-current relationship in at least one vessel. These known ion exchange methods are either continuous or discontinuous, and take place in a fluidized or compact bed. The difficulties arising from the practical application of these various procedures relate more particularly to the resin circulation and are caused primarily by the necessity of discharging the more or less saturated resins during the exchange by taking advantage of the components eliminated from the liquid phase flowing in counter-current relationship, thus gradually reducing the efficiency of the process. As a rule, the liquid phase circulation is discontinued to permit the transfer of the saturated resins by pumping. Other methods involving a continuous transfer of resins are objectionable in that they are ill-suited for transferring one fraction of the resins as a function of their specific granulometry, since the resins having the same granulometry tend to lie in the same contact area. Even in case of initially homogeneous resin granulometry, this granulometry is rapidly modified by wear or attrition.
In an attempt to cope with these various drawbacks, a known proposition consisted in transferring the solid phase, notably the ion exchange resins, by blowing compressed air, but this obviously involves the use of an additional source of energy and is attended by the risk of clogging the transfer piping and developing foam, in case the liquid to be treated contains surface-active substances.
Another known method utilizes an exchange tower comprising a series of contact zones separated by baffle plates providing a central aperture between these contact zones to permit the counter-current flow of the liquid to be treated, and also of the ion exchange resins, the latter being recycled from one zone to another by pumping an output flowing in closed-circuit conditions in both zones. This method is also attended by various inconveniences. Thus, the magnitude of the recycled output involves a local state of hydraulic unbalance at the level of the central aperture between the baffle plates. This strongly disturbed zone is highly detrimental to the ion exchange operation. This recycling from one zone to another implies the use of filter elements which are difficult to maintain. Finally, the use of several pumps is required.